Borehole mining is a technique for extracting minerals from sub-terranean deposits through a borehole with minimal disturbance to the surface. A mining tool containing a water supply conduit and a water jet nozzle, and a slurry pump is lowered down through a borehole to the deposit. The water jet (cutting jet) produced by delivering water under high pressure to the nozzle is used to cut or erode the mineral bearing ore and to create a slurry. The slurry is pumped by the downhole slurry pump to the surface for mineral extraction. As the ore is removed from the borehole, a cavity is created underground. When the mining is complete, the tailings from the extraction process are pumped back into the cavity to reduce the disturbance to the formation surrounding the cavity.
Borehole mining is used to extract a wide variety of minerals such as coal in general, including lignite, uranium, phosphate, gold, kaolin or any other minerals which are found in soft rock formations. The technique works best where a high pressure water jet may be used to cut rocks.
Only a fraction of Canada's extensive tar sand deposits can be extracted using current technology. Shallow tar sands deposits are generally removed by open pit mining techniques. In deep deposits, bitumen can be extracted from tar sands by mining tar sands with conventional tunnel mining techniques or by injecting steam into the tar sands formation and recovering a liquified bitumen from wells by pumping. Between the shallow and deep formation lie enormous reserves of bitumen-laden tar sands, but these cannot be economically extracted by the conventional techniques.
Tar sands are a mixture of sands, clays, water and bitumen; where the sand particles are surrounded by a water film in the sands, clays and bitumen fabric. In the borehole mining technique, the water jet loosens the sand particles from the tar sands fabric and frees the bitumen from the sands. Being nearly naturally buoyant and having nearly the same specific gravity as water, the freed bitumen goes into suspension in the water medium in the cavity. As the clean sand falls to the bottom of the cavity, most of the bitumen attaches itself to air bubbles and rises to the surface in the cavity as froth, whereas a fraction of bitumen that is still attached to the sands falls to the bottom of the cavity and may be lost in the sand.
Ideally, the cutting jet will separate each particle of sand from its neighbours and frees the bitumen. In practice, however, the tar sands deposit will be cut by the water jet and broken into chunks of various sizes, from single particles to large clumps. These clumps then require further cutting action by the water jet to free the bitumen. The most efficient mining technique will be the one that breaks up the tar sands most completely in a single operation.
In the normal borehole mining technique, most of the jetted water in the cavity was removed leaving an air-filled cavity. This was done to give the water jet maximum effective cutting range. However, the air-filled cavity has two major disadvantages. First, the slurry must be pumped under substantial pressure to lift it from the cavity horizon to the surface. The power required for the downhole pump increases as the depth of the deposit increases. At some depth, it is no longer practical or economical to pump the slurry as either the pump size becomes enormously large or a number of pumping stages may have to be applied. Second, when the ore deposit does not have sufficient strength to support the walls of the cavity, subsidence results. Subsidence interferes with the mining process by blocking the slurry pump inlet with large chunks, diluting the ore with overburden and possibly collapsing the cavity.
One way of reducing the downhole pumping requirements and the tendency for the ore to subside, is to fill the cavity to the surface with water, thus providing hydraulic pressure to support the cavity walls. The major drawback of this approach has been the reduction of the water jet's effective cutting range when operating submerged. Traditionally, the water jet nozzle is located on the mining tool which must fit down through the borehole. As the cavity size increases during the mining process, the cavity wall being cut recedes farther from the cutting jet nozzle. Eventually, the jet can no longer reach the cavity wall with enough cutting force to create the slurry. In an air-filled cavity, this distance is on the order of 1,000 times the nozzle diameter. Under water, this distance is reduced to approximately 100 times the nozzle diameter for the same cutting pressure (depending on the ore being mined and the pressure inside of the nozzle). This is a major impediment to the success of borehole mining since the size of the cavity created and therefore the amount of ore removed from the cavity is reduced substantially. This renders the technique uneconomic. To make the system economic, the cavity diameter needs to be maximized.
Previous efforts have been made to extend the range of the submerged water jet by shrouding the jet with an air shield (Flow Technology Report No. 199, August, 1981). This increased the effective range of the jet somewhat but added the complexity and power requirements of a high pressure air compressor. Another technique considered for extending the range of the jet is to simply increase the nozzle size and water flow rate. However, a modest increase in effective range is offset by a substantial increase in required pump horsepower and tool size. Finally, increased cutting jet nozzle pressure does not significantly extend the range of the jet under water.
The prior art describes a variety of extendable/erectable semi-flexible drill stems. In U.S. Pat. No. 4,437,706 issued Mar. 20, 1984 to Gulf Canada Ltd. (Johnson), there is described an apparatus for hydraulic mining of tar sands which includes a cutting nozzle, means for moving the nozzle towards the face as it erodes and recovering means for the separated material. The manner in which the jet is advanced toward the face involves a plurality of jets, with the forward component for the nozzle being at least one jet having a forward angle.
U.S. Pat. No. 2,258,001 to L. C. Chamberlain issued Oct. 7, 1941 shows an apparatus for drilling wells which includes a flexible housing conduit to which is attached a nozzle in the form of a jet. The flexible conduit is formed of spiral metal ribbon.
U.S. Pat. No. 4,007,797 issued Feb. 15, 1977 to Jeter describes a device which includes a housing that can be moved through a borehole and anchored in a desired location for drilling a lateral hole. The housing includes a drill string guide or conductor having a bendable lower end. The drill string and conductor can be retracted into the housing when the lateral hole has been drilled, or a portion of the drill string may be left in the lateral hole as a drain pipe. In the drawings, there can be seen portions of a bendable conductor which included individual links arranged in chain fashion, with each link being hingedly connected to the adjacent link so that each link may pivot relative to the adjacent link about a hinge pin centreline or axis. A flexible tension member urges the link to curve above the hinge pin axis.
U.S. Pat. No. 4,444,276 issued Apr. 24, 1984 to Peterson, Jr. shows a network of flexible tubes or pipes connected to a drill pipe which tubes or pipes are spring biased to flare outwardly such that fluid may be pumped and directed to underground formations.
U.S. Pat. No. 449,459 issued Mar. 31, 1891 to Addison describes a groove cutting machine for oil or gas wells. The structure includes a flexible chain which is adapted to be forced out through an aperture in the side of the casing. At the free or outer end of the claim is a cutting implement. The flexible chain comprises a series of central links, each having a transverse tongue at one end and a transverse groove at the other. Shoulders are cut away upon the front or upper sides of the tongue to permit the chain to bend in an upward or forward position.
U.S. Pat. No. 1,367,042 issued Feb. 1, 1921 to Granville discloses a drilling apparatus which includes a flexible pipe and flexible drive shaft such that holes are drilled at right angles from a well. The flexible drive shaft consisting of a set of links whose connections form a series of universal joints is disclosed. Sucker rods are attached to one end and a drill is attached to the other end of the pipe. Water is fed into the shaft and that pressure drives the drill forward. The drill assembly is pulled back up by exerting a pull on the sucker rods.
U.S. Pat. No. 1,424,109 issued Jul. 25, 1922 to McBride describes a bendable drilling device for horizontal drilling which includes a hollow flexible shaft carrying a bit which shaft carries a flexible hose for conducting water to the bit.
U.S. Pat. No. 2,516,421 issued Jul. 25, 1950 to Robertson discloses a flexible shaft which permits lateral drilling. Means are provided to advance and retract the flexible shaft and a drill head attached thereto.
U.S. Pat. No. 3,191,697 issued Jun. 29, 1965 to Haines discloses another tool for horizontal or lateral drilling which may be extended or retracted.
U S. Pat. No. 4,051,908 issued Oct. 4, 1977 to Driver discloses another system for horizontal drilling.
U.S. Pat. No. 4,577,703 issued Mar. 25, 1986 to Cyriacy et al discloses yet another system for lateral drilling which includes a flexible drilling shaft formed by a steel spiral. A pressure hose extends within the shaft to feed fluid for drilling.
U.S. Pat. No. 4,640,362 issued Feb. 3, 1987 to Schellstede and U.S. Pat. No. 4,658,916 issued Apr. 21, 1987 to Bond describe other lateral drilling system.
Once again there is seen in U.S. Pat. No. 4,658,916 issued Apr. 21, 1987 to Bond a system for drilling lateral holes. Concentric, counterwound spring shafts operating in conjunction with a rotary drive source and a guide housing to direct the rotating, bendable shaft down and outward in a radial direction.
In U.S. Pat. No. 4,674,579 issued Jun. 23, 1987 to Geller et al, there is disclosed on apparatus and method including an offset head fluid drilling and reaming apparatus wherein the drill is maneuverable and has means for remote sensing of orientation and depth. The apparatus is used for drilling unconsolidated material by the use of jet cutting techniques therethrough. Electronic guidance means permits the formation of a hole in a predetermined path or to follow an existing utility line.